This invention relates to cordierite type ceramics and more particularly relates to a method for producing a low thermal expansion ceramic of essentially 100 percent cordierite solid solution.
The low expansion characteristics of cordierite (2MgO; 2Al.sub.2 O.sub.3 ; 5SiO.sub.2 ; 13.8 weight percent MgO, 34.9 weight percent Al.sub.2 O.sub.3, and 51.3 weight percent SiO.sub.2) and cordierite - based ceramics have been known for a long time. Industrial cordierite ceramic bodies came into use during the period 1917 to 1927. Commercial cordierite bodies produced in Europe and the United States are listed in a 1961 publication by Tyrell et al entitled "Synthetic Cordierite," Bureau of Mines Bulletin, 594, U.S. Government Printing Office (1961). The cited thermal expansion values range from about 1100 ppm to 2400 ppm in the range 25.degree. C. to 800.degree. C. The authors performed a comprehensive study of the devitrification of glasses in the MAS system and established the region of best cordierite formation for the system.
However, the authors gave no thermal expansion data for their devitrified glass compositions. Instead, they mixed their material with ball clay to produce fired ceramic articles with linear thermal expansions in the range of about 1800 ppm from 25.degree. C. to 800.degree. C. Since they were primarily interested in the dielectric properties of these ceramics, 1800 ppm thermal expansion was apparently satisfactory for such applications.
Recent interest in the use of ceramics as replacements for metals and high temperature alloys in high temperature applications such as gas turbine engine components has focused attention on the need for low thermal expansion ceramic bodies having corresponding high thermal shock resistance. Among the candidates for such applications are lithium aluminum silicate-based ceramics and magnesium aluminum silicate-based ceramics, with MAS based compositions presently being preferred for their good mechanical properties and corrosion resistance to road and marine salt and sulphorous combustion products.
It is therefore felt that an MAS-based ceramic body having low thermal expansion and satisfactory levels of other physical properties such as density and strength would represent an advancement in the art.